1. Field of the invention
The resent invention relates to an illumination system for endoscopes for medical and industrial use.
2. Description of Related Art
Endoscopes for use with external light sources are typically provided with at least one light guide fiber bundle for guiding light from the external light source to an distal end of the endoscope and illuminating an object in an internal organ or in the interior of hollow structural body. The distribution of illumination light provided by such a light guide fiber bundle does not always cover a view field of the endoscope which has a wide angle of view. In order for the endoscope to cover a wide angle of view field, diverging lens systems are incorporated so as to provide an appropriate distribution of illumination light. By using a concave lens system or a convex lens system as such a diverging lens system light beams from the light guide fiber bundle diverges to illuminate a wide angle of view field.
While the use of these diverging lens system provides illumination for a wide angle of view field, nevertheless, because the illumination system is relatively stationary with respect to an objective lens system, it can not change the distribution of illumination according to angles of sight. For example, in the case where a pair of illumination systems Si are disposed symmetrically with respect to and on diametrically opposite sides of an objective lens system So as shown in FIG. 5A, the illumination systems provides the distribution of illumination as shown in FIG. 5B. In this instance, an object at a relatively short distance is less illuminated at the center area of the view field of the objective lens system So as compared with in the peripheral area far from the optical axis of the objective lens system. Consequently, the distribution of illumination is not uniform over the view field of the objective lens system, providing an image having low brightness at the center area. In the case where a single illumination system Si is disposed on one side of an objective lens system So, the illumination system provides the distribution of illumination as shown in FIG. 6. In this instance, an object at an angle with respect to the optical axis of an objective lens system So is, on one hand, sufficiently illuminated at a relatively short distance and, on the other hand, less illuminated at a relatively long distance. As a result, an image of the object has a low brightness at one side thereof.